The present invention generally relates to an information processing device such as a microcomputer including a read only memory. More particularly, the present invention is directed to an information processing device which has functions of generating a signal used for determining whether the contents of a ROM such as program data have correctly been stored and preventing a third person from reading out or decoding the contents of the ROM.
An information processing device such as a microcomputer equipped with a ROM is widely used. Conventionally, it is required to test a microcomputer as to whether a program has correctly been written into the built-in ROM. For this requirement, there is a need for a means for reading out the contents of the ROM to an external device. In this case, there is a possibility that a third person decodes a stored program from the readout contents of the ROM. From this point of view, normally, a microcomputer is provided with a protection means for preventing a third person from decoding the contents of a ROM.
Japanese Laid-Open Patent Application No. 60-91464 discloses a program readout function which is disabled after reading out the contents of a program memory (ROM) and testing the same. However, there is a need for a specific procedure for disabling the program readout function.
Japanese Laid-Open Patent Application No. 60-83157 discloses a cipher process of shifting or rotating the bit position of readout program data. Japanese Laid-Open Patent Application No. 60-83158 discloses a process of performing a logic AND or OR operation on readout program data. However, there is a possibility that a third person may decode the processed program data with ease, because even when readout program data is processed for every eight bits, the number of possible combinations of conversion is only 256. On the other hand, a complex conversion process increases hardware and is not disable.